I suspect finding anyone inside the US who is sane and willing to blow themselves up is very hard or they would have done so long time ago. Thus the plans to use airlines and send bombers from outside.

Recently they have stopped people from using bombs in the US because the people have used the internet and the FBI or some other agency provided fake bombs to the suspected terrorists. I do believe one or two of these cases have been from US citizens. So they are trying, but luckily we caught them before they did something horrible. So finding people in the US is not as difficult as it seems.